At one customer the write rate to the hard disks is very slow. Is a virtual 32bit UCS system running on a 64bit KVM host.
On the server:
srv:~# dd if=/dev/zero of=/home/foo bs=1M count=100 && rm /home/foo 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 0.0876205 s, 1.2 GB/s
But on the VM it looks like this
root@vm:~# dd if=/dev/zero of=/home/foo bs=1M count=100 && rm /home/foo 100+0 records in 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 67.1148 s, 1.6 MB/s
- correct driver (virtio) was installed
- customer had some customizations of kernel parameters, but all of them were recommended for VMs (e.g. elevator=noop)
- no special IO load on the VM or on the host
- only VM running on the host (so no side effects)
- Internal timing also fails, as the duration of the above test was also noticeable (0.08s vs. 67s!)
- a little bit performance loss is normal with VMs, but not to this extent
- Test installed a 32bit UCS version --> no error found
customer installed a 64bit kernel in the VM -> running
Not a nice and especially not upgradeable solution, but it worked then. System has to be moved to 64bit at some point anyway.